Various materials are used as nuclear fuels for nuclear reactors including ceramic compounds of uranium, plutonium and thorium with particularly preferred compounds being uranium oxide, plutonium oxide, thorium oxide and mixtures thereof. An especially preferred nuclear fuel for use in nuclear reactors is uranium dioxide.
Uranium dioxide is produced commercially as a fine, fairly porous powder, which cannot be used directly as nuclear fuel. It is not a free-flowing powder, but is one which clumps and agglomerates, making it difficult to pack in reactor tubes to the desired density.
The specific composition of a given commercial uranium dioxide powder may also prevent it from being used directly as a nuclear fuel. Uranium dioxide is an exception to the law of definite proportions since "UO.sub.2 " actually denotes a single, stable phase that may vary in composition from UO.sub.1.7 to UO.sub.2.25. Because thermal conductivity decreases with increasing O/U ratios, uranium dioxide having as low an O/U ratio as possible is preferred. However, since uranium dioxide powder oxidizes easily in air and absorbs moisture readily, the O/U ratio of this powder is significantly in excess of that acceptable for fuel.
A number of methods have been used to make UO.sub.2 suitable as nuclear fuel. Presently, the most common method is to die press the powder into cylindrically-shaped green bodies, or compacts, of specific size, which compacts are then sintered.
The various organic or plastic binders that are commonly used for purposes of promoting the production of compacts of powdered materials in preparation for sintering are, however, not useful in application to nuclear fuel because they tend to contaminate the interior of the sintered body with impurities such as hydrides. These binders are normally converted to gases during the sintering step and the gases must be removed requiring special apparatus or procedures. Further, on decomposition, prior art binder materials usually leave deposits of organic substances in the equipment used to sinter the article, complicating maintenance of that equipment. Still further, conventional carbon-base binders would leave a carbon residue in nuclear fuel because they are fired in a reducing atmosphere in which polymeric materials are pyrolyzed.
To a considerable degree, these shortcomings of the prior art have been met and overcome through the invention disclosed and claimed in U.S. Pat. No. 4,061,700, granted to Gallivan on Dec. 6, 1977, and assigned to the assignee hereof. In accordance with that invention, a binder of ammonium uranyl carbonate or corresponding bicarbonate or carbamate is employed to hold the green body or compact together through the handling and processing to the final sintered stage. In more specific terms, green bodies or compacts are produced in accordance with that invention by contacting a particulate mass of UO.sub.2, for example, with ammonium bicarbonate and producing thereby a uniform mixture containing about five percent ammonium uranyl carbonate which following compaction is of density of about 30 to 70% of theoretical, but may be as high as 90%, depending upon the pressing force applied in producing the compact. When formed in a conventional batch pressing operation such as that involving an hydrometallurgical press, these green bodies are strong enough to result in relatively high yields of acceptable products. They cannot withstand nearly as well, however, the force conditions involved in continuous production rotary press operations.
The invention of the above-referenced copending patent application Ser. No. 273,900 provides a superior binder which enables the production of nuclear fuel powder compacts of very high tensile strength or an unusual combination of high tensile strength and plasticity which leads to high yields of compact products in rotary press continuous operations. As previously indicated, that invention involves either addition of ammonium carbonate, bicarbonate or carbamate to nuclear fuel particulate material or the synthesis of it, in situ, and reaction thereof with the fuel to form the corresponding ammonium uranyl compound. Thereafter, that process involves bringing an amine compound into contact with the previously formed ammonium uranyl compound under conditions resulting in liberation of ammonia therefrom with apparent formation of an amine compound corresponding to the original ammonium compound, i.e., amine displacement of ammonia.